1. Field of the Invention
The present invention relates generally to floating offshore platforms. In more specific aspects, the present invention relates to multi-sided floating offshore platforms and methods for constructing such platforms, associated therewith.
2. Description of the Related Art
Offshore platforms are used for processing well fluid from subsea wells. Early offshore structures were supported from the bottom or sea floor. Sea floor supported platforms are still often used in shallow water. When the wells are depleted, however, most governments require that the structure be removed. These bottom supported platforms, being embedded in the sea floor, are not reused, but rather are scrapped at considerable expense after one use. The removal costs are particularly high because these platforms are normally too large to be lifted out of the water, and therefore must be cut up and dumped in approved offshore deep water dumping sites.
Floating offshore platforms are utilized in deeper water. One type of device that has been developed for use in deep and ultra deep water is a deep draft cassion vessel, also known as a spar platform. Such spar-type platforms generally have an elongate cassion hull having an extremely deep kneel draft typically greater than 500 feet. The spar supports an upper deck above the ocean surface and is moored using catenary anchor lines attached to the hull and to seabed anchors. Risers generally extend down from a moon pool in the hull of the spar platform to the ocean floor. The hull of the typical spar platform is generally cylindrically shaped, typically formed of a large series of curved plates positioned in a circular fashion and having a perpendicular radial plane which passes through the isocenter of the hull to form a cylindrical structure. This cylindrical design is used to reduce the severity of the shedding of vortices caused by the ocean currents and to more efficiently resist the hydrostatic pressures. These deep water floating platforms are very costly, usually over $40 million, thus, their use has been restricted to generally only large offshore field developments. Recognized, therefore, is the need for an inexpensive method of constructing such offshore floating platforms. Recognized also is that flat-panel oil tanker-type construction, whether from new construction or through use of existing oil tankers, which can be built in a low-cost tanker shipyard is the lowest cost per ton type of hull construction.
In the not too recent past, oceanic shipments of oil were made primarily in single-skin tankers. In the typical tanker design, mid-ship cargo sections of the tanker are divided by longitudinal and transverse bulkheads into a series of port and starboard side tanks and a center tank. The outer hull plating of the ship forms the outer shell of the side tanks. Similarly, the bottom plating forms the lower shell of the central tanks. Thus, no ballast tanks, void spaces or the like are present between the hull plating and the tanks containing the oil. The defect of such “single-skin” tankers is that any damage to the hull will typically cause the oil in the corresponding tanks to leak, possibly causing damage to coastlines, wildlife, and fisheries. Increasing public awareness of this defect in single-skin tankers and of the fragile nature of the Earth's ecosystem has resulted in substantial worldwide attention to the use of single-skin tankers. Correspondingly, many single-skin oil tankers are going to the scrap yard prematurely, not because they are old, but because they are single-skinned. A fear of many Americans is that a single-skinned oil tanker will run aground like the Exxon Valdez did and will cause billions of dollars of damage to the environment and will ruin tourist beaches. The resulting damage to the economy can be enormous. The United States has thus mandated the phase out of single-skinned tankers in U.S. waters. Other countries are also following the United States lead. The result is a lot of oil tankers are heading to the scrap yards that have good condition oil cargo tanks.
Since an oil tanker carries oil in the cargo tanks and since all the cargo tanks are usually filled and emptied at the same time, the internal surfaces of the oil tankers are almost always coated with oil or an oil film, protecting them from corrosion. A nitrogen blanket is also kept on top of the oil in the tankers protecting all internal surfaces of the oil tanker above the oil from corrosion. The companies that own these tankers usually keep the external coatings on these tankers to a high standard and keep external corrosion to a minimum. The resulting steel on these oil tankers is usually in excellent condition and that steel can serve for many more years as an offshore platform. The scrap value of a very large single skin oil tanker today would be from $5 to $10 million. Double sided and double bottomed oil tankers may also be a good candidate for conversion since they might not need additional internal water tight bulkheads, but they may be as expense to convert as new construction.